Generally described, gas turbine engines are designed to operate with fuels having certain characteristics relating to heating values. The heating value of the fuel may be referred to as the gross caloric value, the gross energy, or the Wobbe Index rating. The heating value generally describes the amount of heat or energy released when the fuel is combusted. The amount of energy released by a fuel being combusted through a fuel nozzle at a given pressure ratio may be more accurately described if the temperature at which the fuel is delivered to the nozzle is taken into account. Such a fuel characteristic may be known as the Modified Wobbe Index rating or MWI rating. Gas turbine engines therefore are designed to operate with fuels that have a specific Modified Wobbe Index rating or with fuels that fall within a range of acceptable Modified Wobbe Index ratings.
The ability to modify or control the temperature of the fuel being delivered to the combustor so as to modify or control the Modified Wobbe Index rating is useful to ensure that the gas turbine engine is using acceptable fuels that promote efficient operation and reduce the risk of combustor damage. Known methods of controlling the incoming fuel temperature generally focus on maintaining the fuel temperature within a given range based upon the varying constituents of the fuel flow. Complex instruments such as gas chromatographs and various types of feedback controls thus may be required to determine the constituents in the fuel and adjust the temperature range accordingly. Moreover, different types of equipment may be used at different stages of operation, i.e., startup, shutdown, other transient events, and steady state. As a result, accurate Modified Wobbe Index rating control may be difficult and/or expensive.
There is therefore a desire for improved systems and methods for adequate Modifying Wobbe Index control without the complexity and expense of known systems. Such improved systems and methods may ensure adequate Modified Wobbe Index control and, hence, adequate overall system performance without requiring complex feedback mechanism and the like.